Q & A

Published: November 8, 1983

Q.

How is yellow butter made from white milk?

A.Butter is made from cream, which is naturally yellowish, as you can see if you let a quart of nonhomogenized whole milk stand for a while in a glass bottle. The yellow component of cream is butterfat, which when extracted from the milk is butter. Butterfat gets its color from carotene, the fat-soluble vitamin A precursor that is also responsible for the color of carrots. Cows get carotene from grass and other vegetable matter they eat. However, not all butterfat is equally yellow. Different breeds of cow differ in their efficiency as converters of carotene to vitamin A, which itself is colorless.

According to Robert P. Niedermeier, chairman of the department of dairy science at the University of Wisconsin, the American population of dairy cows was once heavily weighted with Jersey and Guernsey cows, both of which produced a rich yellow butter. Today there is a larger proportion of Holsteins, which have much less carotene in the butterfat.

Butter made from Holstein milk is much paler than Jersey or Guernsey butter. Since the dairy industry believes consumers prefer a rich yellow butter, the butter is often colored with vegetable colorings to make it yellower.

Q. A friend claims that house plants placed outdoors during thunderstorms will become healthier because of nitrogen released into the atmosphere by lightning. Is this true?

A.Lightning does not release nitrogen into the atmosphere. The atmosphere is about 80 percent nitrogen already. But lightning may ''fix'' nitrogen - that is, cause atmospheric nitrogen, an inert gas of no use to most plants, to join other substances in water-soluble compounds that fall in the rain and are available to the roots of plants. By one estimate, about eight million tons of fixed nitrogen are thought to fall to earth each year in rain.

However, there is considerable doubt whether lightning is really a major producer of fixed nitrogen. There seems to be just as much nitrogen in precipitation in the winter, when lightning is rare, as in summer, when lightning is frequent. Most nitrogen fixing in the atmosphere is apparently performed by extraterrestrial radiation at very high altitudes. In any case, there seem to be no really strong a priori grounds for thinking that lightning would improve the health of house plants.

Readers are invited to submit questions about science to Questions, Science Times, The New York Times, 229 West 43d Street, New York, N.Y. 10036. Questions of general interest will be answered in this column, but requests for medical advice cannot be honored and unpublished letters cannot be answered individually.